Opposites must be mutually paired, so the same feature can be the
opposite of at most one other feature. Consider that if you had a
Variable v and and Function f and called x.setScope(f) you won't know to
add it to localVariables or inputVariables. So editing that side makes
no sense. Instead you might define an operation that's implemented by
inspecting the eContainer of the variable and testing if it's an
instance of Function...